Among various light sources, high-density-discharge (HID) lamps exhibit the best combination of the high luminous efficacy and good color rendition with the high power compact source characteristics. HID lamps have been used in many applications, such as wide area floodlighting, stage, studio, and entertainment lighting to UV lamps.
The use of a high frequency electronic ballast can reduce the size and the weight of the ballast and improve the system efficacy. This feature is especially attractive for low wattage HID lamps because the overall lighting system is expected to be of small size. Moreover, as the operating frequency increases, the re-ignition and extinction peaks disappear, resulting in a longer lamp lifetime. The load characteristic of a HID lamp can be approximated as a resistor and the lamp (power) factor approaches unity. There is no flickering effect and the stroboscopic effect in the light output and the light lumen can be improved. However, the operation of high pressure HID lamps with high-frequency current waveforms is offset by the occurrence of standing pressure waves (acoustic resonance). This acoustic resonance can lead to changes in arc position and light color or to unstable arcs. Instability in the arcs can sometimes cause the arcs to extinguish.
The common explanation for acoustic resonance is that the periodic power input from the modulated discharge current causes pressure fluctuations in the gas volume of the lamp. If the power frequency is at or close to an eigenfrequency of the lamp, traveling pressure waves will appear. These waves travel towards and reflect on the discharge tube wall. The result is standing waves with large amplitudes. The strong oscillations in the gas density can distort the discharge path, which in turn distorts the heat input that drives the pressure wave (W. Yan, Y. K. E. Ho, and S. Y. R. Hui, “Stability study and control methods for small-wattage high-intensity-discharge (HID) lamps,” IEEE Transactions on Industry Applications, vol. 37, no. 5, pp. 1522–1530, September–October 2001). The lamp eigenfrequencies depend on arc vessel geometry, gas filling and gas thermodynamic state variables (such as pressure, temperature and gas density).